Multi-structure thermally trimmable resistors

ABSTRACT

A method for arranging a plurality of thermally isolated microstructures over at least one cavity, each of the microstructures housing at least part of a thermally-trimmable resistor, the thermally-trimmable resistor having at least a functional resistor, the method comprising: providing pairs of facing microstructures; grouping together sets of pairs of facing microstructures, each of the sets having at least one pair of facing microstructures; and arranging microstructures within a given set to have each microstructure exposed to heat from a same number of facing, side, and diagonal neighbors of microstructures from a same resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent Applicationbearing Ser. No. 60/899,648 filed on Feb. 6, 2007 and entitled“Multi-Structure Thermally Trimmable Resistors”.

TECHNICAL FIELD

The present application relates to the field of thermally-trimmableresistors in thermally isolated microstructures, and more specifically,to the layout of multiple microstructures housing thethermally-trimmable resistors on a single substrate.

BACKGROUND OF THE INVENTION

Prior art on thermally-trimmable resistors addresses trimming of suchresistors housed in thermally-isolated microstructures. Themicrostructures offer substantial thermal isolation, allowing themicrostructure to be raised to a high temperature using a minimal amountof power, while the temperature of the surrounding chip remains at a lowtemperature. Typical thermal isolation for cantilevers or membranes usedin thermally-trimmable resistors is tens of degrees Kelvin oftemperature rise per mW dissipated in the microstructure. For example ifa microstructure has thermal isolation 50K/mW, then 20 mW dissipated ina heater-resistor in that microstructure, would raise the localon-microstructure temperature by 1000° C., which would result in thermaltrimming of a functional resistor also housed in that samemicrostructure. Note that the heater-resistor may or may not be the sameresistor as the functional thermally-trimmable resistor, and may or maynot be made of the same materials as the functional thermally-trimmableresistor.

In many cases of practical manufacture of thermally-trimmable resistors,it may be advantageous to use more than one microstructure to house asingle functional resistor having a specific target resistance value.For example, one may want to use one or more cantilever-shapedmicrostructure(s) of a particular standard size, to createthermally-trimmable functional resistors having different resistancevalues. For example, in such a case the cantilever size may berestricted due to limitations in the manufacturing technology (e.g.stress in the films, time needed for the microstructure release etch,mechanical robustness of the microstructure as a function of its size,and/or a fixed range of sheet resistance of the resistor film material).Thus, one may want to electrically connect the functional resistancetraces from more than one cantilever, in series or parallel, and treatthe resulting multiplicity as one device, thermally-trimming them allsimultaneously with common trimming signals applied to theheater-resistors of each cantilever.

In some typical cases of thermal trimming, the heater-resistors are alsothermally-trimmable, and in some cases are subject to failure(open-circuit), when subjected to high power and resulting hightemperatures. Note that in cases where the heater-resistor andfunctional resistor are not the same, the temperature within thefunctional resistor is always somewhat less than the temperature withinthe heater-resistor, because the heater resistor is the source of theheat. Device failure can be brought about by excessive temperature andtypically the trimming is limited by failure in the heater-resistor. Forexample in a single cantilever-shaped microstructure, where a separateheater-resistor and functional resistor are both polysilicon thin films,the trim-down range of the functional-resistor may be greater than 40%,and is limited beyond this point by open-circuiting of theheater-resistor.

Typically, the “trim range” or “trim-down range” refers to the specifiedmaximum induced resistance change downwards (decreasing the resistancefrom its as-manufactured value) at the point where trimming ceases,usually as a result of failure of the heater-resistor (or aggregateheater, in the case where more than one microstructure is used). In manycases, due to the connectivity of the heater-resistors in an aggregatecircuit, when one of the heaters fails (becomes open-circuited), it maydisable (for a variety of reasons) any further heating (signaling theend of trimming). Barring severe manufacturing defects affecting aheater in a specific microstructure, the first heater-resistor to failis typically in the “hottest” microstructure. Under normal operation,the hottest microstructure should also contain the functional resistanceportion trimmed furthest down, meaning that all other microstructureshave not reached their full trim-down potential. In effect, the hottestmicrostructure limits the overall adjustment range of the aggregatethermally-trimmable resistor.

In the case of a multi-microstructure or multi-cantilever resistor, ifall of the microstructures/cantilevers were identically-shaped, withidentical thermal isolation, and if all of the heater-resistors hadidentical resistance, then ideally all of the functional resistancetraces could experience the same temperatures over time, and trimidentically in unison. However, in practice, even if all of thematerials and shapes and resistances were initially identical(initially, before any trim-heating signals are applied), if themicro-structures are positioned near each other in a silicon chip, theheat from trim-heating signals will be shared, to a non-zero extent,causing spatial non-uniformities in temperature, and causing unequaltemperatures experienced by the (otherwise-identical) microstructures.

Typically, deep trim-downs require the highest trimming temperatures,and one may not raise the heater-temperatures indefinitely—eventually,when higher and higher temperatures are reached, the heater-resistor islikely to eventually fail, giving an open-circuit. Therefore, certainmicrostructures are likely to be closer to failure, and theircorresponding functional resistor traces are likely to be trimmed downfurther, than their neighboring microstructures.

With such non-uniformities in temperature, each microstructure mayexperience a different trimming temperature, and thus different trimmingbehavior.

SUMMARY OF THE INVENTION

Non-uniformities in temperature and in trimming behavior are likely tooccur in an array of microstructures where the position of anyindividual microstructure is not symmetric with respect to its neighborsin a closely-spaced group, or in other words, where the position of anyindividual microstructure with respect to its neighbors in aclosely-spaced group is not equivalent to the position of the othermicrostructures with respect to neighbors within the same group.

Therefore, in any instance where the microstructures are close enough toeach other that the heat is shared (meaning that the heat dissipated ineach microstructure raises the temperature in neighboringmicrostructures), the principles described below herein ofmicrostructure positioning should be applied. By using the arrangementssuggested here, we intend to avoid “hot-microstructures” and to minimizetemperature differences between the microstructures composing afunctional thermally-trimmable resistor. While it is natural that withina given single microstructure there may be significant spatialtemperature variations, the intent is that each microstructure have amaximum temperature and a spatial temperature profile as close aspossible to those of the other microstructures which are part of thesame functional thermally-trimmable resistor. In general, we intend toavoid and minimize differences and asymmetries in heating betweenmicrostructures, (asymmetries beyond those caused by random orunavoidable process-induced non-uniformities).

If it is impossible to avoid having significant temperature differencesbetween the microstructures in a given resistor, then a small number(fraction) of “cold-microstructures”, (among a larger fraction of hottermicrostructures whose temperatures are relatively closer together), willgive a better trimming range than a small number (fraction) of“hot-microstructures”, among a larger fraction of colder microstructureswhose temperatures are relatively closer together. This is because inthe case where a small fraction of the microstructures are “cold”, thetrim range will benefit from the larger number of hottermicrostructures. In the case where a small fraction of themicrostructures are “hot”, the rest of the (colder) microstructures willlose substantial trim range since they don't reach the highertemperatures required for deep trim-downs before the heaters begin tofail.

The principles of hot-microstructure avoidance are mostly independent ofthe method of thermal isolation and the shapes of the microstructuresbut can be used in combination with various thermal isolation techniquesand shapes/sizes of microstructures. As long as there is some efficiencyor advantage to be gained by positioning the microstructures in closeproximity to each other (as opposed to just spreading them randomlyaround the surface of the substrate), and as long as the microstructureseach have enough thermal isolation from the surrounding heat-sinks thatthose closely-proximal microstructures can share each other's heat, thenthe principles of symmetry apply in order to make that heat-sharingreciprocal (each shares the same heating from its neighbors).

In accordance with a first broad aspect of the present invention, thereis provided a method for arranging a plurality of thermally isolatedmicrostructures over at least one cavity, each of the microstructureshousing at least part of a thermally-trimmable resistor, thethermally-trimmable resistor having at least a functional resistor, themethod comprising: providing pairs of facing microstructures; groupingtogether sets of pairs of facing microstructures, each of the setshaving at least one pair of facing microstructures; and arrangingmicrostructures within a given set to have each microstructure exposedto heat from a same number of facing, side, and diagonal neighbors ofmicrostructures from a same resistor.

In accordance with a second broad aspect of the present invention, thereis provided a method for arranging a plurality of thermally isolatedmicrostructures over at least one cavity, each of the microstructureshousing at least part of a thermally-trimmable resistor, thethermally-trimmable resistor having at least a functional resistor, themethod comprising: providing pairs of facing microstructures; groupingtogether sets of pairs of facing microstructures, each of the setshaving at least three pairs of facing microstructures; and arrangingmicrostructures within a given set to minimize a temperature differencebetween microstructures for a same resistor, the temperature differencecaused by a spatial relationship and a number of neighboringmicrostructures for a same resistor from whom heat is shared, a diagonalneighbor providing less heat than a facing or side neighbor.

In accordance with a third broad aspect of the present invention, thereis provided a method for arranging a plurality of thermally isolatedmicrostructures over at least one cavity, each of the microstructureshousing at least part of a thermally-trimmable resistor, the methodcomprising: providing pairs of facing microstructures; grouping togethersets of pairs of facing microstructures, each of the sets having atleast three pairs of facing microstructures; and arrangingmicrostructures within a given set for a same resistor to have a smallernumber of microstructures exposed to less heat than microstructuresexposed to more heat, a level of heat being a result of a spatialrelationship and a number of neighboring microstructures for a sameresistor from whom heat is shared, a diagonal neighbor providing lessheat than a facing or side neighbor.

In accordance with a fourth broad aspect of the present invention, thereis provided a substrate comprising a plurality of thermally isolatedmicrostructures each housing at least part of a thermally-trimmableresistor, the thermally-trimmable resistor having at least a functionalresistor, the thermally isolated microstructures provided in pairs offacing microstructures, the pairs grouped together into sets, each ofthe sets having at least one pair of facing microstructures, and eachset being arranged for heat-sharing, each microstructure in a given setexposed to heat from a same number of facing, side, and diagonalneighbors of microstructures from a same resistor.

In accordance with a fifth broad aspect of the present invention, thereis provided a substrate comprising a plurality of thermally isolatedmicrostructures each housing at least part of a thermally-trimmableresistor, the thermally-trimmable resistor having at least a functionalresistor, the thermally isolated microstructures being arranged in setsof pairs of facing microstructures for heat-sharing, the microstructuresin a given set arranged to minimize a temperature difference betweenmicrostructures for a same resistor, the temperature difference causedby a spatial relationship and a number of neighboring microstructuresfor a same resistor from whom heat is shared, a diagonal neighborproviding less heat than a facing or side neighbor, each set having atleast three pairs of facing microstructures.

In accordance with a sixth broad aspect of the present invention, thereis provided a substrate comprising a plurality of thermally isolatedmicrostructures each housing at least part of a thermally-trimmableresistor, the thermally-trimmable resistor having at least a functionalresistor, the thermally isolated microstructures being arranged in setsof pairs of facing microstructures for heat-sharing, the microstructuresarranged within a given set to have a smaller number of microstructuresexposed to less heat than microstructures exposed to more heat for asame resistor, a level of heat being a result of a spatial relationshipand a number of neighboring microstructures for a same resistor fromwhom heat is shared, a diagonal neighbor providing less heat than afacing or side neighbor.

In this specification, the term “neighbor” is intended to mean amicrostructure that is beside, in front, or diagonal to anothermicrostructure. The term “hot microstructure” is intended to mean amicrostructure receiving more heat from neighboring microstructures thanother surrounding microstructures. The term “cold microstructure” isintended to mean a microstructure receiving less heat from neighboringmicrostructures than other surrounding microstructures. Note that theheater-resistor may or may not be the same resistor as thethermally-trimmable resistor, and may or may not be made of the samematerials as the thermally-trimmable resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 a is a single file three microstructures illustrating the“hot-microstructure” effect;

FIG. 1 b is a set of three pairs of facing microstructures illustratingthe “hot-microstructure” effect;

FIG. 1 c is a non-symmetric set of six microstructures illustrating the“hot-microstructure” effect;

FIG. 1 d is a set of five pairs of facing microstructures illustratingthe “hot-microstructure” effect;

FIGS. 2 a to 2 c are examples of microstructure designs that do notexperience the “hot-microstructure” effect;

FIG. 2 d shows three pairs of microstructures more widely spaced than inFIGS. 2 a to 2 c, in order to reduce and/or eliminate the“hot-microstructure” effect;

FIG. 2 e is a layout using spacing for four pairs of facingmicrostructures to reduce and/or eliminate the “hot-microstructure”effect;

FIG. 2 f is a layout using dummy microstructures for four pairs offacing microstructures to reduce and/or eliminate the“hot-microstructure” effect;

FIG. 2 g is a layout using a heat absorbing baffle for four pairs offacing microstructures to reduce and/or eliminate the“hot-microstructure” effect;

FIGS. 3 a to 3 d are arrangements with interleaved microstructures for1:1 ratios of the number of microstructures per resistor, in order toreduce and/or eliminate the “hot-microstructure” effect; and

FIGS. 4 a to 4 d are arrangements with interleaved microstructures forspecific non-1:1 ratios of the number of microstructures per resistor,in order to reduce or eliminate the “hot-microstructure” effect.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION

Consider FIG. 1 a, depicting an array of 3 cantilever-shapedmicrostructures, single-file, all on the same side of a cavity, with allheaters being trim-pulsed simultaneously. In the figures, showingcantilever-shaped microstructures, each of the microstructures shows twosymbols each representing an electrical resistance, a heater-resistanceportion and a functional resistance portion, which are electrically (butnot thermally) isolated from each other. Note, these resistance symbolsare not intended to represent actual shapes of resistance lines in themicrostructures, rather only the presence of resistance elements. Eachresistance portion may or may not be a part of a larger resistorconsisting of more than one microstructure—with the functionalresistance portion being a part of a larger functional resistor, andwith the heater resistance portion being a part of a larger heaterresistor. Alternatively, certain embodiments may include only afunctional resistance portion and no heater resistance portion. In thiscase, heating is done via the functional resistance portion.

If the three heaters in FIG. 1 a each receive identical heating power(say, P_(o)/3), such that the total power dissipated in the entire groupof 3 microstructures is P_(o)), the heat dissipated in the 3 heaterswill be partially shared. Each microstructure will be heated by thepower dissipated within itself, and will to a lesser extent alsoexperience temperature rise due to power dissipated in its neighboringmicrostructure(s). Thus, if the spacing between microstructures is smallenough, the steady-state temperature of each microstructure in the groupof 3, will be greater than that of a single isolated microstructurereceiving power P_(o)/3. Since the central microstructure has two suchneighbors, its temperature will be higher than the other twomicrostructures (who each only have one such neighbor). Thus thisconfiguration gives spatial temperature differences among the 3microstructures, with the microstructure in the center experiencing thehighest temperature of the three. Note: this may be further exacerbatedby changes in resistance over temperature (TCR effects) and/or dynamictrimming effects of the heater-resistor, which can further increase thepower dissipated in the “hot” microstructure, potentially furthermodifying the temperature differences among the microstructures.

The same type of “hot-microstructure” difficulty applies to FIG. 1 b (6cantilevers, arranged in pairs on opposite sides of a single cavity).The pair of cantilevers facing each other in the center will experiencethe highest temperatures of the three pairs, since they each have onefacing neighbor, two side-neighbors and two diagonal neighbors, and arereceiving heat from their neighbors on three out of four sides, whilethe four corner cantilevers each have only one facing neighbor, one sideneighbor, and one diagonal neighbor, and are receiving heat from theirneighbors only on two out of four sides.

In general, where there are two rows of microstructures arranged onopposite sides of a rectangular cavity, the most significant heatsharing must be a function of proximity and is dominated bycontributions from facing or side-neighbors. Both of these will causemore heat sharing than a diagonal neighbor. Whether it is facing or sideneighbors which contribute the most to temperature rise depends on thedistance-to-neighbor in each of the two directions (facing vs side).

FIG. 1 c shows a non-symmetric array of six microstructures, which wasalso experimentally tested, and demonstrates a severe decrease in trimrange. While a single cantilever had a trim-down range of greater than40%, the group of cantilevers in FIG. 1 c can be trimmed down by only˜25%, and the first heater to become open-circuited is always in themicrostructure indicated (circled) in the figure—the one which achievesthe highest temperature, since it alone has the highest number ofimmediate (facing or side) neighbors of any microstructure in the group,as well as a diagonal neighbor.

As another example, consider an array of 10 cantilever-shapedmicrostructures arranged in two rows of 5, opposite each other above asingle bulk-micro-machined cavity in a silicon chip (see FIG. 1 d), withall heaters being trim-pulsed simultaneously. If these 10 heaters eachreceive identical heating power (say, P_(o)/10, such that the totalpower dissipated in the entire group of 10 microstructures is P_(o)),the heat dissipated in the 10 heaters will be partially shared, and theaverage temperature in the 10 microstructures will be greater than itwould be if a single isolated microstructure received P_(o)/10.Furthermore, the temperature will vary spatially among the 10microstructures. The four individual microstructures on the four cornerswill experience the lowest temperatures (of the 10 microstructures), andthe two microstructures facing each other at the center of the arraywill experience the highest temperatures (of the 10 microstructures).

Table 1 shows the results for trim-down percentages of individualmicrostructures in the 10-microstructure array shown in FIG. 1 d. Themicrostructures (and their embedded heater-resistors and functionalresistors) were all designed to be identical, and common trim-heatingelectrical pulses were applied to all 10 heaters simultaneously. Asshown in Table 1, six different trim-downs were applied, (labeled tr#1to tr#6), to increasing trim-down amounts, as measured by the overallseries resistance of the 10 functional resistors. Clearly the twocentral microstructures trim down the most (indicating that they havereached the highest temperatures), while the four corner microstructureshave trimmed down the least.

A single microstructure is by its nature not prone to such“hot-microstructure” effects. In a pair of identically-designedmicrostructures, positioned side-by-side (as in FIG. 2 a), each of thetwo microstructures experiences the same effect from the sharing of heatfrom its neighbor, provided the spacing to the cavity above and below isnot asymmetric with respect to the microstructures—thus is not prone to“hot-microstructure” effects. Similarly, in a pair ofidentically-designed microstructures, positioned facing each other (asin FIG. 2 b), each of the two microstructures experiences the sameeffect from the sharing of heat from its neighbor, provided the spacingto the cavity above and below is not asymmetric with respect to themicrostructures—thus is not prone to “hot-microstructure” effects.

In a group of four identically-designed microstructures, positioned intwo groups of two, facing each other (as in FIG. 2 c), each of the fourmicrostructures experiences the same effect from the sharing of heatfrom its neighbors, provided the spacing to the cavity above and below,is not asymmetric with respect to the microstructures—thus is not proneto “hot-microstructure” effects.

In cases where one wants to use only 3 microstructures to implement aspecific target resistance value, it would be advantageous (from thepoint of view of avoiding “hot-microstructures”) to position the 3within a symmetric group of 4, and apply the trim-heating signals to allfour heaters. In one embodiment, the fourth microstructure (a dummy) isidentical to the others including that it has identical functional andheater resistors as the other microstructures, and has identical thermalconduction paths for heat to flow to and from it, to imitate the heatflow to and from the other three active microstructures, except that itsfunctional resistor is not electrically connected as part of the overallfunctional resistor composed of the other three functional resistorsegments. This may include dummy electrical lines, to imitate the heatconduction of the other three functional resistor segments, but whichare electrically disconnected from those other three functional resistorsegments.

Note that in a group of four microstructures, uniformly spaced in a row(similarly to FIG. 1 a), the central microstructure(s) will be “hotmicrostructures” with respect to the two microstructures positioned atthe ends of the row—(unless the spacing between the microstructures islarge enough that the sharing of heat becomes negligible). The sameproblem applies to a group of 8 microstructures, in two rows facing eachother (similarly to FIG. 1 b)—the central four microstructures will be“hot” with respect to the microstructures positioned at the four cornersof the array.

If one must use more than four microstructures, then it is desirable togroup them into sets of 2 or 4 such that heat-sharing from one set tothe next is minimized, such as by increasing the spacing between the set(as shown in FIGS. 2 d, 2 e), or by inserting a heat-absorbing bafflebetween the sets (FIG. 2 g), or by placing the sets of 2 or 4microstructures in separate dedicated cavities. FIG. 2 d shows threepairs (and three sets) of facing microstructures, spaced farther apartthan in FIG. 1 b, in order to reduce or eliminate the “hotmicrostructure” effect in the middle pair of microstructures. FIG. 2 eis an arrangement using extra separation between the two sets of fourmicrostructures, in order to reduce or eliminate temperature differencesbetween the four microstructures in the middle vs. the fourmicrostructures on the corners of the cavity. FIG. 2 f is an arrangementusing “dummy” microstructures in order to reduce or eliminatetemperature differences between the four microstructures in the middlevs. the four microstructures on the corners of the cavity. Note that inFIG. 2 f, the dummy microstructures are not heated while heating signalsare applied to any of the other heaters. These dummy microstructures inFIG. 2 f are shown as being identical to other microstructures, but canalso be of arbitrary shape and size, as well as being composed ofdifferent materials. FIG. 2 g is an arrangement using a heat-absorbingbaffle between the two cavities (indicated by dashed lines), in order toreduce or eliminate temperature differences between the fourmicrostructures in the middle of the figure vs. the four microstructureson the outer corners of the figure. The cases depicted in FIGS. 2 d, 2e, 2 f, 2 g may also be seen as techniques to effectively separate alarger group of microstructures into sets of 4, intending to benefitfrom the symmetry inherent in groups of 4, as described in FIG. 2 c.

Indeed, Table 2 shows the results of experimental trim-downs (similar tothose described above for Table 1), for the structure depicted in FIG. 2f, where the two central microstructures are “dummies”—identical to theothers except that their heaters do not receive trim-heating signals,and their functional resistor is not connected (not part of the measuredfunctional resistance consisting of the series connection of the other 8resistors). The trim-down amounts of the 8 functional resistors embeddedin each of the 8 microstructures are much more uniform than was found inTable 1.

One may also make changes to the layout and/or materials in themicrostructures, to increase the thermal isolation of the coldermicrostructures and/or decrease the thermal isolation of the hottermicrostructures, such that the actual temperature differences areminimized. The thermal isolation could be relatively increased by anumber of means, for example reducing the width or thickness ofheat-conducting materials connecting the microstructure to any nearbyheat sink(s). By increasing the thermal isolation of a specificmicrostructure in this way (by changing the heat-conduction pathsthrough the solid connections to the microstructure), one increases thetemperature reached by that specific microstructure (for a given powerdissipated within that microstructure), thus increasing the net heatflow from that microstructure to other neighboring microstructures.Similarly, if one decreases the thermal isolation of a specificmicrostructure, one reduces the temperature reached by that specificmicrostructure (for a given power dissipated within thatmicrostructure), thus decreasing the net heat flow from thatmicrostructure to other neighboring microstructures (increasing net heatfrom to that microstructure from other neighboring microstructures).

Overall thermal isolation is affected by several physical phenomenarelated to the various “paths” by which heat flows away from themicrostructure. These include heat conduction through the solid arms ofthe microstructure out to the main substrate, heat conduction into thegas in the cavity, and heat radiation away from the microstructure. Inthe embodiments described herein, assuming that the cavity above andbelow the microstructure are far enough away, the major heat conductionpath is out through the microstructure arms. It is mostly affected bythe width and thickness of the polysilicon and metal traces that go outthe arms onto the substrate, since these are the most-heat-conductivematerials in the arms. So increasing thermal isolation means reducingheat flow out through the arms with the goal of raising the temperatureof the microstructure, for a given amount of power dissipated within it.If the temperature is raised, then it is expected that the neighboringmicrostructures will feel that temperature rise by heat flow from it.Note that the main mechanisms by which the neighboring microstructuresare heated are radiation and conduction in the surrounding gas—and it isexpected that these would not be changed by changing heat-conductionpath out through the arms.

One may deliberately increase the amount of power dissipated in thecolder microstructures, such as by reducing their heat-resistance, oradding an auxiliary heat source. One may also deliberately decrease theamount of power dissipated in the hotter microstructures of a givenfunctional resistor. This can be accomplished by, for example,partitioning the heater-resistor in a hotter microstructure into twoportions, one portion dissipating power elsewhere (away from the set ofmicrostructures), leaving a smaller percentage of the power beingdissipated within the microstructure. One may also partition theheater-resistors within a given functional resistor, such that the two(or more) groups are not heated at the same time. This is done byconnecting together the microstructure heaters of a subset ofmicrostructures in a set so that they receive heating signals together,while the remainder of the microstructures in the same set do notreceive the heating signals until another time, whereby the subsets aredisconnected from each other so that one subset can be given heatingsignals without heating the other subset(s).

The above analysis applies to avoiding “hot-microstructures” in a singlethermally-trimmable resistor. Alternatively, in many cases of design ofthermally-trimmable resistors, it is desired to include two or morefunctional thermally-trimmable resistors on a single chip. In this case,some advantages may be attained by co-arrangement of themicrostructures. For example:

FIG. 3 a shows a set of microstructures including twothermally-trimmable resistors in a single cavity, where R1 and R2 eachare composed of 4 microstructures, alternating across the cavity suchthat when trimming signals are applied to the heaters of one of R1, R2(not both simultaneously), heat sharing is minimized between themicrostructures. In this case, the coldest microstructures have only one(diagonal) neighbor, while the hottest microstructures have two diagonalneighbors. The temperature difference between hottest and coldest maystill exist, but relatively small (compared to, say the temperaturedifferences in FIG. 1 b or 1 d). Table 3 shows the trim-down percentagesfor a configuration as shown in FIG. 3 a—showing some non-uniformity,but far less than in Table 1.

FIG. 3 b shows an alternative configuration for a set ofmicrostructures. Again two thermally-trimmable resistors are housed in asingle cavity, where R1 and R2 are each composed of 4 microstructures.The four inner microstructures (R1) all have the same spatialrelationships with their neighbors (within R1, which will all receiveheat simultaneously), and the four outer microstructures (R2) all havethe same spatial relationships with their neighbors within R2 (i.e.which comprises two pairs of facing microstructures, each pair being farfrom the other such that there is negligible heat-sharing from one pairto the other).

FIG. 3 c again shows two thermally-trimmable resistors in a singlecavity, but here they are arranged in alternating pairs for a sameresistor, such that each resistor has an “outer” pair and an “inner”pair, separated from each other by a pair from the other resistor (whichshould be enough separation to avoid substantial heat sharing betweenthe two pairs). Note, if more area is available, each pair can be placedin a separate cavity.

FIG. 3 d shows a case similar to FIG. 3 a, where R1 and R2 are eachcomposed of 3 microstructures, alternating across the cavity such thatwithin a given resistor there are only diagonal neighbors (no facing ordirect side neighbors). Larger numbers of microstructures are alsoamenable to these principles, as will be understood by a person skilledin the art.

The above examples (FIGS. 3 a-3 d) are effective when one can implementthe two functional resistance values R1, R2 in an equal number ofmicrostructures, such as when the resistance ratio R1:R2 is relativelynot too far from 1:1. Other challenges can arise when it is desired toimplement two thermally-trimmable resistors having substantiallydifferent resistance values, while using same or very similarmicrostructures (each housing same or similar functional resistancevalues). In such cases, where it is desired to keep the number ofmicrostructures in each resistor proportional to the resistance ratio,it may be difficult to alternate or interleave the microstructures.Whatever the reason for it being desirable to have specific non-1:1ratios of the numbers of microstructures, the techniques below may beapplied to minimize hot-microstructure degradation effects.

For example, a 1:2 ratio of microstructures in a given set can beimplemented as shown in FIG. 4 a, where in R2 the two pairs of facingmicrostructures are positioned symmetrically on the two ends of thecavity such that each microstructure has the same relationship to all ofits neighbors within R2, and each of the two pairs of facingmicrostructures is relatively well separated from the other.

However, a 1:3 ratio in a given set becomes more problematic, since in aplanar rectangular geometry, for R2 it is impossible to position threepairs of facing microstructures such that each microstructure has thesame relationship to all of its neighbors, and since there is only onepair from R1 to interleave between two pairs from R2. In practice,adding a facing pair of dummy microstructures would accomplish the wideseparation of the three pairs in R2, at the cost of larger area, as wasdone in FIG. 2 f.

Also, in general, if area is available, the influence of neighbors canbe reduced and temperature differences can be decreased by increasingthe distance between microstructures, or by placing each pair (or groupof 4) of microstructures in its own separate cavity.

In the case where it is important to avoid the use of dummymicrostructures (such as in the interests of area-efficiency), or othertechniques for separation that involve increasing the area used, thenFIG. 4 b shows a possible arrangement. Since it is most interesting toavoid large differences in temperatures between the coldest and hottestmicrostructures, and since R1 does not have enough microstructures toinhabit all four corners, the presence of coldest microstructures cannotbe avoided. Thus, one attempts to substantially reduce the temperaturedifferences primarily by reducing the temperatures of the hottestmicrostructures in a given set. The arrangement shown in FIG. 4 baccomplishes this, to a certain extent, because in R2 the threedifferent configurations have not dramatically different neighborarrangements. The top-right microstructure has one facing neighbor andone diagonal neighbor; the top-left microstructure has one facingneighbor and one side neighbor (likely hotter than the top-rightmicrostructure, because a side neighbor delivers more heat than adiagonal neighbor); and the left-side second-from-top microstructure hasone side neighbor and two diagonal neighbors (no facing neighbor). Thedifference between hottest and coldest is due only to the differencebetween a facing neighbor and two diagonal neighbors, or to thedifference between a side neighbor and a diagonal neighbor, or to thedifference between a facing neighbor vs a side neighbor and a diagonalneighbor.

In another example, if the desired ratio of microstructures is 4:10 in agiven set, there is again no way to use the R1 pairs to separate R2 intogroups of 2 or 4. In such a case, as mentioned above, it is better tohave a relatively small number of “cold” microstructures than arelatively small number of “hot” microstructures. Thus the arrangementdepicted in FIG. 4 c gives a suitable solution. Even though the pair ofmicrostructures in the center will be “colder” than the groups of 4microstructures, during thermal trimming of R2, thetemperature-difference-induced trim range reduction will be low becauseonly two microstructures are affected.

Increasing the number of microstructures beyond those described hereinmay have the benefit of averaging out the effects of hotmicrostructures. For example, if FIG. 1 d is extended to more pairs ofmicrostructures, e.g. 7 pairs, the central pair of microstructures isstill likely to be the hottest, but likely not so much hotter than thepairs adjacent to the center, thus reducing the temperature differencesand alleviating somewhat the “hot-microstructure” effect. However, suchan increase in number of microstructures would require increased area,and thus may be counterproductive to certain principles of efficientanalog electronics design. For the purpose of design where device areais not a constraint, or designs requiring higher power-carryingcapability, it can be considered.

Note that the microstructures do not need to be shaped as cantileverssuch as are depicted in the figures—many different shapes ofmicrostructures are subject to the principles described herein.

The embodiments of the invention described above are intended to beexemplary only. The scope of the invention is therefore intended to belimited solely by the scope of the appended claims.

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TABLE 3

1.-21. (canceled)
 22. A substrate comprising a plurality of thermallyisolated microstructures each housing at least part of athermally-trimmable resistor, said thermally-trimmable resistor havingat least a functional resistor, said thermally isolated microstructuresprovided in pairs of facing microstructures, said pairs grouped togetherinto sets, each of said sets having at least one pair of facingmicrostructures, and each set being arranged for heat-sharing, eachmicrostructure in a given set exposed to heat from a same number offacing, side, and diagonal neighbors of microstructures from a sameresistor during a trimming procedure.
 23. A substrate as claimed inclaim 22, wherein said sets are separated, separated sets having lowerheat-sharing capabilities between sets than between microstructures of asame set.
 24. A substrate as claimed in claim 23, wherein said separatedsets comprise spacing between said sets to reduce a heat-sharing effectbetween microstructures of neighboring sets, whereby spacing betweenmicrostructures in a same set is smaller than spacing between sets. 25.A substrate as claimed in claim 23, wherein said separated sets comprisea pair of facing dummy microstructures between neighboring sets, saiddummy microstructures not receiving trim-heating signals during trimmingof said microstructures in said neighboring sets.
 26. A substrate asclaimed in claim 23, wherein said separated sets comprise aheat-absorbing baffle between neighboring sets.
 27. A substrate asclaimed in claim 23, wherein said separated sets comprise a separatededicated cavity for each set.
 28. A substrate as claimed in claim 22,wherein a given set comprises an additional microstructure, which is adummy microstructure, when an odd number of microstructures is present,said dummy microstructure paired with one of said microstructures into apair of facing microstructures, said dummy microstructure comprising aheater receiving trim-heating signals during trimming of saidmicrostructures and a dummy functional resistor that is not electricallyconnected to said functional resistor of said microstructures.
 29. Asubstrate as claimed in claim 22, wherein said pairs of facingmicrostructures are for a same resistor, and said sets comprise pairs offacing microstructures for a first resistor alternated with pairs offacing microstructures for a second resistor.
 30. A substrate as claimedin claim 22, wherein said pairs of facing microstructures are for a sameresistor, and said sets comprise sets of one pair of facingmicrostructures for a first resistor alternated with sets of two pairsof facing microstructures for a second resistor.
 31. A substrate asclaimed in claim 22, wherein said microstructures in a given set arecontiguous and have substantially equal spacing between side neighborsand substantially equal spacing between front neighbors.
 32. A substratecomprising a plurality of thermally isolated microstructures eachhousing at least part of a thermally-trimmable resistor, saidthermally-trimmable resistor having at least a functional resistor, saidthermally isolated microstructures being arranged in sets of pairs offacing microstructures for heat-sharing, said microstructures in a givenset arranged to minimize a temperature difference betweenmicrostructures for a same resistor during a trimming procedure, saidtemperature difference caused by a spatial relationship and a number ofneighboring microstructures for a same resistor from whom heat isshared, a diagonal neighbor providing less heat than a facing or sideneighbor, each set having at least three pairs of facingmicrostructures.
 33. A substrate as claimed in claim 32, wherein saidsets comprise microstructures for a same resistor arranged to have somemicrostructures sharing heat from two diagonal neighbors and othermicrostructures sharing heat from a single diagonal neighbor, andwherein no heat is shared by a front or side neighbor.
 34. A substrateas claimed in claim 32, wherein said sets comprise a smaller number ofmicrostructures exposed to less heat from its neighbors compared toother microstructures in a given set, than microstructures exposed tomore heat from its neighbors compared to other microstructures in thegiven set.
 35. A substrate as claimed in claim 32, wherein said setscomprise an increased thermal isolation to microstructures in a set thatare colder than other microstructures for a same resistor underequivalent thermal isolation conditions.
 36. A substrate as claimed inclaim 35, wherein said increased thermal isolation comprises a reducedwidth of heat-conducting materials connecting the colder microstructuresto nearby heat sinks for a same resistor in a set.
 37. A substrate asclaimed in claim 32, wherein said sets comprise an auxiliary heat sourceto increase an amount of power dissipated in colder microstructurescompared to hotter microstructures for a same resistor in a set.
 38. Asubstrate as claimed in claim 32, wherein said sets comprise aheater-resistor partitioned into at least two portions for hottermicrostructures for a same resistor in a given set, one of said at leasttwo portions dissipating power away from said given set.
 39. A substrateas claimed in claim 32, wherein said sets comprise a partitioned heaterresistor for a same functional resistor that exposes selected ones ofmicrostructures in a given set to heat while not exposing othermicrostructures of the given set to heat.
 40. A substrate as claimed inclaim 32, wherein each microstructure in said pairs of microstructureshas a microstructure heater, said microstructure heater being part of aheater resistor used for heating said functional resistor.
 41. Asubstrate as claimed in claim 40, wherein said microstructure heater ina hotter microstructure in a given set is partitioned into at least twoportions that can be heated separately, to reduce peak power dissipationwithin said hotter microstructure.
 42. A substrate as claimed in claim40, wherein a given set has a plurality of said microstructure heatersconnected together to receive heating signals to dissipate heat inselected ones of microstructures while not dissipating heat in othermicrostructures of the given set from said heating signals.
 43. Asubstrate comprising a plurality of thermally isolated microstructureseach housing at least part of a thermally-trimmable resistor, saidthermally-trimmable resistor having at least a functional resistor, saidthermally isolated microstructures being arranged in sets of pairs offacing microstructures for heat-sharing, said microstructures arrangedwithin a given set to have a smaller number of microstructures exposedto less heat from its neighbors compared to other microstructures in thegiven set, than microstructures exposed to more heat from its neighborscompared to other microstructures in the given set during a trimmingprocedure, a level of heat being a result of a spatial relationship anda number of neighboring microstructures for a same resistor from whomheat is shared, a diagonal neighbor providing less heat than a facing orside neighbor.